Drug-Related Hypothyroidism

Scientists at the University of Illinois at Urbana-Champaign have found that a drug commonly used to treat dogs for bacterial infections induces hypothyroidism. The antibiotic trimethoprim/sulfamethoxazole (T/S) consistently blocks production of thyroid hormones in dogs causing hypothyroidism. Hypothyroidism caused by short-term administration of T/S is reversible when treatment is discontinued.

"We are very excited about our findings in the study's conclusive demonstration of the significant effect of trimethoprim/ sulfamethoxazole on thyroid gland function in the dog," said Karen L. Campbell, DVM, MS, principal investigator. This Morris Animal Foundation-funded study is entitled, "The Affect of Potentiated Sulfonamides on Thyroid Physiology in the Dog."

Hypothyroidism is the most common endocrine disease affecting dogs. Signs of hypothyroidism include reduced activity, hair loss, recurrent infections, infertility, intolerance of cold, and weight gain. Blood tests are commonly used to measure the concentration of thyroid hormones. Dogs with low levels of thyroid hormone are often treated with life-long supplementation of thyroid hormones. While thyroid blood tests are easy to do, they are not always an accurate reflection of a dog's ability to produce thyroid hormones.

The "gold standard" for evaluation of thyroid function is the TSH simulation test. According to Dr. Campbell, drugs may interfere with the response of the thyroid gland to the TSH simulation test resulting in an inaccurate test.

Dr. Campbell's research found that dogs being treated with T/S at a dose of 3 0 mg/kg twice daily with develop iatrogenic hypothyroidism within one to six weeks of treatment. All thyroid hormones are affected with marked depression of total T4, free T4, total T3, free T3 and reverse T3 and marked elevation of endogenous TSH concentrations. Dogs with low serum thyroid levels should be checked for a history of T/S administration. Retesting of thyroid hormones should be done after the dog has been off T/S for a minimum of three weeks. Thyroid biopsies and radionuclide thyroid imaging can be used to distinguish between T/S induced hypothyroidism and naturally-occurring hypothyroidism.

Dr. Campbell recommends: "The decision of veterinarians to treat an animal with T/S should include the consideration of the effects of the antibiotic on thyroid glands; alternative antibiotics should be selected whenever possible." She adds that additional studies are needed to determine if other commonly used sulfa drugs have a similar adverse effect on thyroid gland function in dogs.

from Animal News, Morris Animal Foundation